(1) Field of the Invention
The present invention relates to an optical transmission method and an optical transmission system for performing repeating transmission of a wavelength division multiplexed (WDM) signal light while amplifying thereof, and, especially to a superimposition technology of a supervisory signal in a case of amplifying a WDM signal light using Raman amplification.
(2) Related Art
For a WDM optical transmission system for transmitting a WDM signal light including a plurality of optical signals of different wavelengths, there has been known a system structure adopted with an optical amplifying-and-repeating transmission system using an optical amplifier as an optical repeater. For the above-mentioned optical amplifier, an erbium doped optical fiber amplifier (EDFA) is generally used, and recently, the use of Raman amplifier together with the EDFA is being greatly reviewed.
A gain of the Raman amplifier has large wavelength dependency, but a gain wavelength characteristic of the Raman amplification can be flattened by the use of a plurality of pumping lights of different oscillation center wavelengths. For example, in the Raman amplifier described in the article, “100 nm bandwidth flat gain Raman amplifiers pumped and gain-equalized by 12-wavelength channel WDM high power laser diodes”, Y. Emori, et al., OFC '99, PD19, 1999 and so forth, approximately 100 nm as a gain wavelength bandwidth of the Raman amplification is secured by adjusting a pumping light power and its oscillation wavelength.
For the WDM optical transmission system adopted with the optical amplifying-and-repeating transmission system, there is known a technology to supervise and control a plurality of optical transmission apparatuses such as an optical sending terminal, an optical repeater, an optical receiving terminal, and the like, by superimposing a supervisory signal on a main signal light to be transferred among each of optical transmission apparatus. For one conventional method for superimposing the supervisory signal on the main signal light in this case, there is known a method for modulating a driving current of a pumping light source of the EDFA in accordance with the supervisory signal. Specifically, a transmission speed of the supervisory signal is set to approximately 10 Mb/s and its superimposition degree is set to approximately 5% when a transmission speed of the main signal light is set as 10 Gb/s.
Considering a case where the above conventional monitor control technology is applied to the WDM optical transmission system constructed by using the optical repeater that uses the EDFA and the Raman amplifier together, which are being reviewed presently, the method for superimposing the supervisory signal on the main signal light is not limited to the method for modulating the pumping light of the EDFA in accordance with the supervisory signal, and a more efficient supervising and controlling technology is expected to be realized. Further, since it can be considered that, in the future, the optical repeater shall be constituted using only the Raman amplifier, it will be useful to realize the supervising and controlling technology capable of coping with the system using such optical repeaters.